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Balancing the Use of Crop Residues for Biofuels with Impacts on Soil and Greenhouse Gases

The use of crop residues as a second-generation source of biofuels may hold potential to help the United States fulfill its 2022 goal production quota outlined in the 2007 Energy Independence and Security Act. Yet, this annual accumulation plays an important role toward maintaining soil organic carbon (SOC) stocks and reducing soil erosion, protecting field health to sustain year-after-year of yields. Adler et al. (2015) use the DayCent biogeochemical model to analyze the costs and benefits of crop residue removal and use based upon its impact on crop yield, SOC content, and N2O emissions, over the course of twenty years. They examined these relationships with respect to a variety of anticipated treatment options, including: a baseline condition with no residue removal, a sample of 50% residue removal without any replacements, 50% residue removal with a nitrogen replacement equivalent to the amount removed, and a 50% residue removal and equivalent application of a high-lignin fermentation byproduct (HLFB). The sites selected were intended to represent a diversity of alternative-power options and costs, climate and soil characteristics, and crop rotation systems across the U.S., to capture a broad overview of potentially influential factors. Adler et al. monitored changes in SOC and N2O brought about by residue removal and conducted a life cycle impact assessment, incorporating greenhouse gas emissions and recoveries, to estimate their global warming intensity. While residue removal and sources of replacement had varied impacts on N2O based on the addition of nitrogen and the climate of the growing area, SOC always decreased with residue removal. However, the overall trade-off between SOC loss and emissions from crop residue removal could be minimized by returning HLFB (which cannot contribute to the ethanol itself) to the land after the refinement process is complete. Returning the high-lignin fraction of crop residue to fields holds potential to help maintain soil quality and nitrogen content, offsetting many of the negative impacts of its removal, to some extent. Further, these benefits hold potential to reduce more greenhouse gas emissions overall by making additional ethanol available as an alternative fuel source, than if crop residues were left on the land and not included in the production process in any way.